The requirements for decreasing size, decreasing weight and increasing performance of electronic devices have been advanced, the degree of integration and density of semiconductor elements (hereinafter also referred to as the element or chip) is increasing year by year, and surface mounting of semiconductor devices (hereinafter also referred to as the package) has also become widespread. With the progress of related technologies of such semiconductor devices, the requirements for a resin composition for encapsulating a semiconductor element have become more severe as well. For example, in a surface mounting process, when a semiconductor device that has absorbed moisture is exposed to a high-temperature atmosphere during a soldering process, cracks are formed, or detachment occurs inside the semiconductor device due to explosive stress of rapidly vaporized steam, and the operation reliability of the semiconductor device is significantly impaired. Furthermore, with the elimination of use of lead, lead-free solder, which has a melting point higher than conventional solder, is used instead. In the case where such lead-free solder is used, it is necessary to increase the temperature during semiconductor mounting by about 20 degrees centigrade as compared with the case where conventional solder is used, and the stress during the aforementioned soldering process becomes more severe. With the spread of the surface mounting technology and use of the lead-free solder, solder resistance in a resin composition for encapsulating a semiconductor becomes one of important technical objects.
Against the background of recent environmental problems, there is a growing social need to eliminate the use of a flame retardant such as a brominated epoxy resin, an antimony oxide or the like, which has been used in the past, and the technology for imparting flame retardance equivalent to the conventional flame retardance has become necessary without the use of these flame retardants. As an alternative flame retardant technology, there has been proposed, for example, a method of adopting a crystalline epoxy resin having a lower viscosity and mixing a larger amount of inorganic filler (for example, see Patent Documents 1 and 2). However, it is hard to mention that solder resistance and flame retardance are sufficient by the method.
Furthermore, in recent years, there has appeared a package having a structure obtained by laminating chips inside one package, or a semiconductor device with a wire diameter thinner than the conventional semiconductor device. Since the thickness of the resin-encapsulated portion in such a semiconductor device is thinned than the conventional semiconductor device, there is the risk of lowering the yield rate in an encapsulating process such that insufficient filling easily occurs, or wire sweep easily occurs during molding. In order to improve flow properties of the resin composition, the use of an epoxy resin or a phenol resin curing agent having a low molecular weight is easily considered, but according to this method, there are defects such that defective transferor stoppage of an equipment (deterioration of handling property) easily occurs during the molding process due to adhesion of the resin composition (tablets), or properties of solder resistance, flame resistance and moldability are impaired due to deteriorated curability in some cases. As described above, it becomes an important object for the resin composition to balance flowability, handling property, solder resistance, flame resistance and moldability due to fine wiring and thinning of a semiconductor device.